Control device for controlling the position of a marine seismic streamer

ABSTRACT

A control device ( 1 ) for controlling the position of a marine streamer comprising; an annular aerofoil ( 2 ), a mount ( 3 ) for mounting the annular aerofoil onto and around the streamer; and control means ( 4   a,    4   b ) for controlling the tilt and/or rotation of the annular aerofoil ( 2 ) whereby to adjust the lateral position and/or depth of the streamer.

[0001] This invention relates to control devices for controlling theposition of a marine streamer such as military towed array or a seismicstreamers.

[0002] Towed arrays or seismic streamers are generally towed behind aship and can be thousands of metres in length. For carrying out aseismic survey, a series of streamers are towed, parallel and at apredetermined depth below the water surface. Devices known as “birds”are used to control the depth of the streamer in the water. However, ingeneral, the birds of prior art designs are not able to correct thetransverse alignment of the streamers, if, for example, a cross currentaffects a streamer's position. There is a desire to be able to controlsuch transverse alignment so as to prevent entangling of adjacentstreamers and to provide an optimal geometry for data gathering.

[0003] Streamers are typically deployed and retrieved by means of awinch. Another problem of many prior art designs is that the birds aretoo cumbersome to allow them to be permanently fixed to the streamers,the birds, necessarily having to be attached or removed from thestreamers at intervals as the streamers are deployed or retrieved.Furthermore, the hydrodynamic stability of prior art designs isquestionable, particularly after a period of man-handling in the field.

[0004] The problem of controlling the transverse alignment of streamersis to some extent alleviated by a proposal in EP-B-0939910 whichproposes a bird having two independently controllable, planar wingsconfigured to provide both lateral and transverse depth control. Thesame device however is designed with a rigid length that precludes itfrom stowage whilst attached to the streamer.

[0005] The present invention aims to alleviate the disadvantages of theprior art designs discussed above.

[0006] In accordance with a first aspect, the present invention there isprovided a control device for controlling the position of a marinestreamer comprising;

[0007] an annular aerofoil,

[0008] a mount for mounting the annular aerofoil onto and around thestreamer; and control means for controlling the tilt and/or rotation ofthe aerofoil whereby to adjust the lateral position and/or depth of thestreamer.

[0009] The mount may conveniently comprise a hollow shaft positionedcentrally of the annular aerofoil and rotatably mountable about alongitudinal axis of the streamer. Alternatively the mount may bepermanently fixed to the streamer and have an outer surface rotatablymounted about the fixed portion of the mount. The mount is preferablyarranged so as not to be concentrically aligned with the annularaerofoil. The annular aerofoil may be connected to the mount by radiallyarranged struts positioned between the mount and the annular aerofoil.The control means typically includes one or more motors for adjustingthe rotational or translational position of the annular aerofoil.

[0010] In some embodiments, the struts may be of a symmetrical aileronshaped cross-section having a plane of symmetry radial to the plane ofthe annulus of the annular aerofoil, and the annular aerofoil may beweighted in a portion of its annulus so as to cause the device, when noother loads are applied, to sit with the struts at a predeterminedrotational position with respect to horizontal. A plurality of devicesweighted at different positions about the annulus of the annularaerofoil may be positioned at a similar distance along the length ofadjacent streamers to aid separation of the streamers due to the liftingeffect of the differently, rotationally positioned aileron struts. Eachcontrol device may be rotated about 2 axes. One substantially inalignment with the longitudinal axis of the streamer and onesubstantially orthogonal thereto (ie to tilt the annular aerofoil in aforward, backward, left or right direction). Movement of the device maythereby be effected in two dimensions, one given by the lift produced bythe aileron struts when the device is rotated and a second produced bythe annular aerofoil when the device is tilted.

[0011] In alternative embodiments the control means may include one ormore ailerons rotatably mounted between the mount and the inner surfaceof the annular aerofoil. Conveniently a single pair of ailerons arerotatably mounted in diametrically opposing positions. Rotation of eachaileron may be carried out independently by a motor. Optionally theailerons may be operatively connected to two protrusions, one extendingradially outwardly from the outer surface of the mount and the otherradially inwardly from the inner surface of the annular aerofoil.Optionally, one of the pair of ailerons is fixed in position where itadjoins the annular aerofoil and free to rotate adjacent the mount.

[0012] When desired, the control means may be configured to permit thepair of ailerons to be repositioned simultaneously and independently,for example, an opposing pair may each be simultaneously tilted-to thesame angle of incidence to provide lift in a single direction withoutrotation. The radial position of the ailerons about the longitudinalaxis of the streamer may equally be adjusted by means of a motor.Rotation of the ailerons' radial position about the longitudinal axisdesirably causes a simultaneous rotation of the annular aerofoil aboutthe same axis. Alternatively, the ailerons are provided within a framework which is freely rotatable within the centre of the annularaerofoil. The framework may, for example, incorporate a cylindricalframe work separated from the inner surface of the annular aerofoil onlyby bearings which permit the annular aileron and cylindrical frame torotate independently and optionally be locked to rotate in unison.

[0013] Controlling motors may conveniently be powered by electricalcables passing through the streamer and connected to a power source onthe marine vehicle carrying the streamers. Alternatively, motors may bebattery powered. The controlling means may be purely electromechanicaland operated manually or alternatively may be connected with acomputerised system configured to manage positioning of the streamerswithin the water. Such a computerised control means may further comprisea system of transducers for monitoring the actual position of thestreamers and the components of the control device.

[0014] It is to be understood that for the purposes of this invention,the term annular aerofoil is intended to include annular rings withvaried cross sectional, aerodynamic profiles. These cross sectionalshapes may range from a simple oval or elliptical shape to more complexaerofoil designs. The annular aerofoil is mounted about the streamerwith its attachment points as near as possible coplanar with its centreof lift, thus minimising the forces required to control its orientation.

[0015] The main components of the control device i.e. the annularaerofoil and, where incorporated, the ailerons, are corrosion resistant,lightweight and hard materials which are known in the art and methods ofmanufacture will no doubt occur to the skilled addressee from hisknowledge of the relevant technological field, and may include, titaniumor aluminium alloys.

[0016] The control device of the invention may be removably attachableto a streamer or, preferably, may be permanently fixed to a streamer.The design of the control devices of the invention are sufficientlyrobust and compact that they may be permanently fixed to a streamer andmay be wound with the streamer onto a winch of appropriate design forconvenient storage and deployment.

[0017] In another aspect, the invention provides a marine streamercomprising;

[0018] an elongate, flexible cable carrying along its length a pluralityof hydrophones and a plurality of control devices for controlling theposition of the marine streamer in water, the control devices beingspaced along the length of the streamer and permanently mountedthereabout and each comprising;

[0019] an annular aerofoil,

[0020] a mount for mounting the annular aerofoil onto and around thestreamer; and

[0021] control means for controlling the tilt and/or rotation of theaerofoil whereby to adjust the lateral position and/or depth of thestreamer.

[0022] The streamer preferably carries electrical cables which carry apower source to the control means. The mount preferably provides for theannular aerofoil to be rotatably mounted to and about the streamer.Other features of the mount may include those features as described inaccordance with the first aspect of the invention. The control means maybe as previously described in accordance with the first aspect of theinvention.

[0023] The control devices are preferably positioned at separations ofbetween 100 and 500 metres along the length of the streamer. Thestreamer may be a military towed array. Optionally, the streamer may bea seismic streamer.

[0024] For the purposes of exemplification, some embodiments of theinvention will now be further described with reference to the followingFigures in which;

[0025]FIG. 1 shows in perspective view an embodiment of a control deviceof the invention.

[0026]FIG. 2 shows in cutaway the embodiment of the control device asshown in claim 1.

[0027]FIG. 3 shows in more detail the componentry of an embodiment of acontrol device similar to that shown schematically in FIGS. 1 and 2;

[0028]FIG. 4 shows an embodiment of marine streamer according to theinvention wound on a winch in a first configuration;

[0029]FIG. 5 shows an embodiment of marine streamer according to theinvention wound on a winch in a second configuration.

[0030] As can be seen in FIGS. 1 and 2, a control device generallydesignated as 1 comprises an annular lift surface 2 having a generallyaerofoil shaped cross section 6. To the centre of the annular aerofoilis a cylindrical mount 3 which surrounds the circumference of a marinestreamer (not shown). The axis of the mount is not concentric with theinner surface of the annular aerofoil. The inner cylinder is fixedlymounted to and around a marine streamer. A pair of ailerons 4 a, 4 b arerotatably mounted about radial axes 5 a, 5 b between the outer surfaceof the mount 3 and two diametrically opposed points with the innersurface of the annular aerofoil. Aileron 4 a is fixedly connected withthe inner surface of the annular aerofoil and rotatable at its endadjacent the mount. Aileron 4 b is rotatable both with respect to theinner surface of the annular aerofoil and the mount.

[0031] The rotational position of each aileron 4 a, 4 b about the axes 5a, 5 b is independently controllable by a control system (not shown).The assembly comprising the ailerons 4 a, 4 b, the axes 5 a and 5 b andthe annular aileron 2 may all be rotatably mounted about thelongitudinal axis of the marine streamer by means of bearings in themount 3. Alternatively the mount may be fixed, the streamer beingrotatable with the control device under the control of the controlmeans.

[0032] An embodiment of the invention as first described above would besimilar in appearance to that shown in FIGS. 1 and 2. However, the twoailerons 4 a, 4 b are each fixed in position both adjacent the mount andthe inner surface of the aileron, the symmetrical plane of each aileronbeing radial to the plane of the annulus of the annular aerofoil, andthe leading edge of each aileron directed in the same direction as theleading edge of the annular aileron. In such embodiments, the mount 3 isconfigured to allow rotation of annular aileron about the streamer.

[0033] Principles of the design of the embodiment illustrated in FIG. 3are now summarised. The design requirements assume the device should beoperable at speeds of up to 30 knots.

[0034] Referring to FIG. 3, the angle of attack of the annular aerofoilis controlled by a servo motor 1 which is directly coupled to a gear box2 which drives an M6 lead screw 3 via a tongue and slot coupling 4, thelead screw is mounted in a double row angular contact ball bearing 5,which is able to deal with both pushing and pulling forces generated bythe lead screw.

[0035] The lead screw nut 6 is circular, and a slide fit in the casing.Vertical forces generated by the connecting rod are transferred throughthe lead screw nut 6 to the casing. The connecting rod 8 is connected atone end to the lead screw nut 6 by means of two pins 7, and at the otherend to a crank ring 9, whose purpose is to convert the push/pull motionprovided by the connecting rod in to rotary motion of the drive tube 10.The crank ring 9 is held in position on the drive tube 10 by a dog pointsocket set screw 11, which engages with a hole in the drive tube 10.

[0036] The forces required from the system to control the ring wingsangle of incidence are calculated in Example 1 below. The forces whichthe proposed system will provide to control the annular aerofoil's angleof incidence are calculated in Example 2 below.

[0037] The aileron system is driven by an electric servo motor 12, themotor is integrated with a reduction gear box 13. The gear box outputshaft is connected to a propeller shaft 15 by a hooks coupling 14. Thepropeller shaft 15 contains a sliding device within it, so that itslength can vary, to accommodate the variation in length required as theaerofoil angle of incidence changes.

[0038] The output end of the propeller shaft is connected to a singlestart worm gear 16 via a second hooks coupling 14. The worm gear ismounted in ball bearings 17, and engages with a 38 tooth gear wheel 18,which is also mounted in roller bearings 19.

[0039] The worm wheel is of conventional design, however the gear wheelspecifically adapted for the purpose by provision of a larger boss thanstandard, an eccentric stub axle turned on one end and there is no holethrough it.

[0040] The worm wheel 16 and gear wheel 18 are mounted on the crank ring9 which controls the angle of incidence of the annular aerofoil, hencethe need for a variable length propeller shaft and two hooks couplings.The advantage of mounting the aileron system so that it moves with theannular aerofoil, is that the position of the aileron relative to theannular aerofoil will not change as the annular aerofoil incidence ischanged.

[0041] The shaft of the potentiometer which senses the aileron position21 is fixed in the hole in the centre of the gear wheel 18, on theopposite side to the eccentric stub axle 20. As the maximum movement ofthe gear wheel for fall aileron incidence in one direction to fullincidence in the other is only 190°, a 360° potentiometer 21 will be asatisfactory transducer.

[0042] The eccentric stub axle 20 is mated to an operating arm 22, whichpasses through a slit in the drive tube 10 which controls the annularaerofoil's incidence. It then passes through a slot in the main wingspar port 29 to the centre of the main wing spar port where it engageswith the aileron drive shaft 23 which has a square drive cut in to itsend. This shaft runs up the main wing spar port via two needle rollerbearings 24 and 25 and a thrust bearing 26 together with a seal 34 toprevent the ingress of water down the port main wing spar, to a pointwhere a square drive on the outboard end of the shaft engages withanother operating arm 27. The latter arm passes out of the main wingspar via a pair of slits, and is then imbedded in the centre of theaileron foil section 28. The aileron foil is made in two halves (thesplit is horizontal along its centre line) which are screwed together.This is to permit the operating arm 27 to be imbedded in it.

[0043] The main wing spar is shown installed in the lower elevation ofFIG. 3. In order to facilitate the requirement of being removable fromthe array and to aid assembly, it is in two halves port 29 and starboard30. In order to transmit its movement to the ring wing, the outboardends have square drives 34 cut in to them, which mate with square holescut in the annular aerofoil. The annular aerofoil is dividedlongitudinally across the top of these holes, so that there are upperand lower halves. The two halves are held together in the same manner asthe clamps for the 64 mm diameter array (although on a larger scale).

[0044] The port side of the main spar 29 is hollow, it contains theaileron drive shaft 23 for the aileron mechanism, described above. Atits inboard end, it has a slot to enable it to be slid over theoperating arm 22 which engages with the eccentric stub axle. Part of theslotted portion of the in board end of the port spar is threaded, thisthread is to accept a 12 mm diameter bolt 31 which is inserted throughthe starboard half of the main spar 30. Within the head of the bolt 31is an ‘O’ 32 which prevents water travelling down the inside of thespar.

[0045] The central body of the unit is made up from five sections. Atthe forward end is a forward end unit 35, this has a front end which isidentical to the forward end of 64 mm array units (female), and carriesa standard connector. The rear end of this unit has a standard 64 mmarray unit female end, but it is open and so does not contain aconnector.

[0046] The next unit is the aileron motor/gearbox unit 36, this unithouses the aileron motor/gearbox, and has 6mm holes running through ittop and bottom on the port side, to carry the wires which run the lengthof the streamer, and the wires to carry power to the annular aerofoilincidence control motor. On the lower starboard side is a further holeto carry the wires from the potentiometers which sense the annularaerofoil and aileron positions.

[0047] The rear of this unit is equipped with a smooth 56 mm diametersection which contains an ‘O’ ring for the central section to seal on,together with a M56×2 (male) threaded portion. The smooth 56 mm diameteralso has a blind M5 screw hole tapped into it together with a recess.This enables a lock washer to be assembled into the recess prior toassembly, and following assembly a M5 cap head screw is assembled toprevent the joint undoing.

[0048] The next unit is the central section 37. This unit is bored fromeither end, both ends carry a plain 56 mm diameter section to seal onthe ‘O’ rings in adjacent sections. These plain sections also havesuitably positioned holes for the M5 locking screws mentioned above.Following the plain sections are M56×2 (female) threaded sections.

[0049] Following the M56 thread either end, the unit is bored out to 58mm diameter. The forward end provides the space needed for the crankring 9 to move through 15° in either direction. The rear space containsthe annular aerofoil potentiometer 38, together with the connecting rod8.

[0050] Both of the 58 mm diameter sections contain sections of aluminium(not shown) which fit and are stuck to the walls of the 58 mm diametersections. These sections have 6 mm diameter holes drilled through them,which line up with the port side wire holes in the aileron motor/gearboxunit, central section and annular aerofoil incidence motor/gearboxsection. This will mean that there are two fully lined 6 mm diameterholes through the complete unit for the passage of wires.

[0051] The two 58 mm diameter sections are separated by a solid section,which has a vertical rectangular hole cut in it (this can be finish cut,following milling, using a shaping machine), this controls the positionof the crank ring in one plane. The solid section provides the metal inwhich the hole for the drive tube is cut, together with an ‘O’ ringgrove on either side. There is also a 6 mm hole through the solidsection top and bottom on the port side for the wires, together with a 4mm hole through on the lower starboard side for the annular aerofoilpotentiometer wires.

[0052] The block 40 locates the annular aerofoil incidence potentiometer38, and is held on to the rear face of the central solid section by twoscrews (not shown). The annular aerofoil incidence potentiometer has a28 (0.5 MOD) tooth gear wheel stuck to its shaft, this mesh with a 120tooth gear wheel which has been cut away to fit the space available, andwhich is fixed to a spigot which is machined in the side of the camring. Over the complete 30° movement of the wing incidence, this willrotate the potentiometer: $\begin{matrix}{\quad {= {{120/28} \times 30\quad {degrees}}}} \\{= {128.6\quad {degrees}}}\end{matrix}$

[0053] So again a single turn potentiometer will be satisfactory.

[0054] The next unit going aft is the annular aerofoil motor gearboxsection 39. This houses the motor gearbox, coupling, bearing and leadscrew. It carries a thread, seal and locking screw similar to those onthe aileron motor/gearbox unit. It contains extensions of the 6 mm wireholes on its port side. Again like the aileron motor/gearbox unit it hasa standard 64 mm diameter male array end at its aft end, without aconnector.

[0055] The final part of the central section is the aft unit 41, thishas a standard 64 mm female end at its forward end, but withoutconnector, the aft part of the unit is a completely standard 64 mm maleend, with connector.

EXAMPLE 1

[0056] Forces Required to Control Annular Aerofoil Angle of Incidence

[0057] In order to minimise the forces required to control the annularaerofoil angle of incidence, the annular aerofoil must be mounted asnear to its centre of lift as practicable. In practice for a normalwing, the centre of lift usually occurs a distance which is 25% of thechord length from the leading edge. The centre of lift does moveslightly according to the angle of incidence. In practice, forsymmetrical aerofoil sections the centre of lift may be assumed to liein a band between 24% and 26% of the chord length from the leading edge,for reasonable angles of incidence.

[0058] Because of manufacturing tolerance, some uncertainty aboutexactly where the centres of lift and drag will be for an annularaerofoil and also to make some allowance for friction within the annularaerofoil pivot and connecting rod bearings; it will be assumed that themaximum driving force required is equivalent to mounting the aerofoil ata point which is 10% of chord away from the actual centre of lift.

[0059] If the annular aerofoil is made so that it has a diameter of 250mm measured from the centre of its chord, then it can be shown that thechord length becomes:

100×250/180=138.9 mm

[0060] Using a nominal plan area of the annular aerofoil of chord xnominal diameter. That is Area=0.10×0.18 metres^(2″).

[0061] The maximum value of C_(L) achieved at 15° is 1.05. Applying thefollowing:

Lift=(C _(L)×Density×Area×Velocity²)/(2×g_(o))

[0062] Then at 30 knots:

Lift=(1.05×1000×1.03×0.25×0.1389×(15.44)²)/(2×9.81) kg. $\begin{matrix}{\quad {= {456.32\quad {{kg}.}}}} \\{= {4476.5\quad {{Newtons}.}}}\end{matrix}$

[0063] For an actual chord of 103 mm, a 10% of chord offset would be:

0.1×(103×250)/180=14.31 mm

[0064] So the torque required to drive the annular aerofoil at maximumincidence at 30 knots is: $\begin{matrix}{{Torque} = {4476.5 \times 0.01431\quad {Newton}\quad {{metres}.}}} \\{= {64.1\quad {{Nm}.}}}\end{matrix}$

EXAMPLE 2

[0065] Forces Generated to Control Annular Aerofoil Angle of Incidence

[0066] The motor selected to control the ring wing angle of attack is aPortescap 28 L 28, this has a maximum intermittent torque rating of 57mNm. The proposed gear box is from the same manufacturer, and is an R32.0 with a reduction of 72.3: 1, and an efficiency of 65%. It has acontinuous torque rating of 4.5 Nm, and a static torque rating of 20 Nm.If the intermittent rating of the motor is multiplied by the gearreduction and the efficiency:

0.57×72.3×0.65=2.679 Nm

[0067] This is well within the gear boxes continuous rated capacity.

[0068] The gear box drives an M6×1.0 screw thread. It is known that totighten the average M6 screw to yield requires 2.8 kgm of torque, andthis will produce a load of 2030 kg.

[0069] Because the screw is mounted in a roller bearing, it isreasonable to assume that the torque required to overcome the frictionin the head end of the screw will sensibly be eliminated. It would alsobe a conservative estimate to assume that half of the friction torque tobe overcome is generated in the head end. This is because the radius atwhich the friction is generated, is larger for the head than on thethread.

[0070] If we consider the screw being done up one turn under theconditions above, but assume it is lifting a weight, rather than justbeing tightened, then: $\begin{matrix}{{{Work}\quad {to}\quad {tighten}\quad {screw}} = {2.8 \times 2p\quad {kgm}}} \\{= {17.593\quad {kgm}}}\end{matrix}$ $\begin{matrix}{{{Mechanical}\quad {work}\quad {done}} = {{2030/0.001}\quad {kgm}}} \\{= {2.03\quad {kgm}}}\end{matrix}$ $\begin{matrix}{{{So}\quad {total}\quad {friction}\quad {torque}} = {17.593 - {2.03\quad {kgm}}}} \\{= {15.563\quad {kgm}}}\end{matrix}$

[0071] So the addition of the ball bearing mounting for the screw willreduce the total work required to: $\begin{matrix}{\quad {{= {{15.563/2} + 2}},{03\quad {kgm}}}} \\{= {9.812\quad {kgm}}}\end{matrix}$

[0072] Or put another way, the torque required to tighten the originalscrew is reduced from 2.8 kgm to: $\begin{matrix}{\quad {= {2.8 \times {9.812/17.593}}}} \\{= {1.562\quad {kgm}}} \\{= {15.32\quad {Nm}}}\end{matrix}$

[0073] Our motor gearbox combination produces 2.679 Nm, so thecalculated push/pull force available from the screw will be:$\begin{matrix}{{2030 \times {2.679/15.32}} = {354.98\quad {kg}}} \\{= {3482.35\quad N}}\end{matrix}$

[0074] Within the proposed design the minimum angle of the connectingrod to the screw axis is 17°, and the minimum effective radius of theconnecting rod is 19.97 mm.

[0075] The effect of the connecting rod being at an angle to the screwis actually to increase the push/pull force within the connecting rod.This is because the roller bearings which, support the vertical loadsapplied to the M6 nut by the connecting rod actually contribute to theload within the connecting rod.

[0076] Given that the screw force is 3482.35 N, the push/pull forcewithin the connecting rod will be: $\begin{matrix}{\quad {= {{3482.35/\cos}\quad 17\quad N}}} \\{= {3641.46\quad N}}\end{matrix}$

[0077] This force acting at a radius of 19.97 mm will produce a torqueon the tube driving the ring wing of: $\begin{matrix}{\quad {= {3641.46 \times {19.97/1000}\quad {Nm}}}} \\{= {72.72\quad {Nm}}}\end{matrix}$

[0078] When compared with the required figure of 64.1 Nm calculated inExample 1 this shows that the system will have approximately 13% moretorque than required, even at 30 knots with a full 15° angle ofincidence.

[0079] Certain commercially available lubricants and anti-seizingcompounds can reduce the coefficient of friction from the region of0.19-0.25 down to 0.05, so it would seem that an expected torque of72.72 Nm may be conservative.

EXAMPLE 3

[0080] Annular Aerofoil Response Time

[0081] From the zero incidence position, the mechanisms response isslightly asymmetrical, requiring 5.69 turns of the screw in onedirection to achieve the full 15° of incidence, and 5.79 turns in theother direction.

[0082] If we look at the slower (5.79 turns) direction. At the loadrequired for 30 knot operation the motor speed will vary between3500-8000 r.p.m. So if we assume an average motor speed of:$\begin{matrix}{\quad {= {{\left( {3500 + 8000} \right)/2}\quad {r.p.m.}}}} \\{= {5750\quad {r.p.m.}}}\end{matrix}$

[0083] Then the time taken to apply 15° of incidence will be:$\begin{matrix}{\quad {= {5.79 \times 72.3 \times {60/5750}\quad {seconds}}}} \\{= {4.37\quad {seconds}}}\end{matrix}$

[0084] At the quoted operational speed of 10 knots, the loads willreduce to 11% of their previous value, and the motor will achieve 8000r.p.m. throughout the movement, so reducing the time to apply 15° ofaerofoil incidence to: $\begin{matrix}{\quad {= {5.79 \times 72.3 \times {60/8000}\quad {seconds}}}} \\{= {3.14\quad {seconds}}}\end{matrix}$

EXAMPLE 4

[0085] Required Aileron Operating Torque

[0086] Two 180 mm diameter annular aerofoil diverters were both rolledabout 30° at 10 knots. Given that the counter weight in each annularaerofoil weighed 1 kg, and that it was situated at 90 mm radius, fromthe centre of the wing. The correcting moment applied when the ring hasbeen rotated 30° is:

1000×90 Sin 30=45000 gram mm

[0087] Given that the counter weight will have been curved (to fit theaerofoil), its centre of gravity will probably be at a slightly smallerradius, so the correcting moment was probably a little less than thecalculated figure.

[0088] If we now scale this figure up to account for the intended largersize of the annular aerofoil: $\begin{matrix}{{{Correcting}\quad {moment}} = {45000 \times \left( {250/180} \right)^{3}\quad {gram}\quad {mm}}} \\{= {120563.27\quad {gram}\quad {mm}}} \\{= {0.12056\quad {kgm}}}\end{matrix}$

[0089] The stiffness of the streamer will vary according to the positionalong the streamer at which the annular aerofoil is placed, the lengthand stiffness of the tow cable used etc.

[0090] It would probably be best if the array is deployed with theannular aerofoil and its aileron feathered, so that the streamer cantake up its natural orientation before control operations begin with theannular aerofoil.

[0091] Due to the uncertainties in the amount of correcting momentrequired, it has been decided to design for a correcting moment of 0.25kgm at 10 knots, this is slightly-more than twice the calculated figureabove.

[0092] When scaled up to 30 knots, this becomes 2.25 kgm.

EXAMPLE 5

[0093] Correcting Moment Generated by the Aileron System

[0094] The aileron has the same chord length as the struts (138.9 mm),and occupies the complete half span between the central control body andthe annular aerofoil. This gives it a span of: $\begin{matrix}{\quad {= {125 - {21/2} - {{64/2}\quad {mm}}}}} \\{= {82.5\quad {mm}}}\end{matrix}$

[0095] It is supported on the annular aerofoil support shaft, and pivotsabout it. The aileron has an aspect ration of: $\begin{matrix}{\quad {= {82.5/138.9}}} \\{= 0.594}\end{matrix}$

[0096] The proximity of the annular aerofoil and the central controlbody to the tips of the aileron, will have a profound effect on itsapparent aspect ratio, making it appear very much larger than itactually is. If we assume that the aileron's effective aspect ratio is2, then the following values of C_(L) may be expected. Increase in Angleof Coefficient of Coefficient of lift for incidence degrees lift 5°incidence increase 20 1.02 0.26 15 0.76 0.24 10 0.52 0.26  5 0.26 0.26

[0097] From which it can be seen that the minimum increase in C_(L) for5° increase in incidence is 0.24 when the angle of incidence isincreased from 10° to 15°. So using this figure when the aileron isgiven 5° of incidence to the annular aerofoil, the additional lift forcegenerated will be: $\begin{matrix}{\quad {= {\left( {C_{L} \times {Density} \times {Area} \times {Velocity}^{2}} \right)/\left( {2 \times g_{o}} \right)}}} \\{= {{\left( {0.24 \times 1000 \times 1.03 \times 0.0825 \times 0.1389 \times (15.44)^{2}} \right)/\left( {2 \times 9.81} \right)}\quad {kg}}} \\{= {34.42\quad {kg}}}\end{matrix}$

[0098] The effective radius at which this force is applied is:$\begin{matrix}{\quad {= {{82.5/2} + {32\quad {mm}}}}} \\{= {73.25\quad {mm}}}\end{matrix}$

[0099] So the effect of applying 5° of incidence to the aileron will beto apply a torque of: $\begin{matrix}{\quad {= {34.42 \times {.07325}\quad {kgm}}}} \\{= {2.52\quad {kgm}}}\end{matrix}$

[0100] to the annular aerofoil. (Target 2.25 kgm)

[0101] Using the ‘C’ Group data sheet coefficients, the C_(L) value for20° is 1.02 (when 15° of incidence is applied to the annular aerofoiltogether with 5° of additional aileron incidence), however this value ofC_(L) is unrealistic when the complete annular aerofoil has a best C_(L)value of 1.05 at 15°, based on projected area only (actual area is threetimes projected area). It would seem reasonable to suppose that as theannular aerofoil is actually a tri-plane, that the proportion of theC_(L) value contributed by the central section is 1/3. If we assume thatthe extra 5° of incidence behaves as normal, then C_(L) will be:$\begin{matrix}{\quad {= {{1.05/3} + 0.24}}} \\{= 0.59}\end{matrix}$

[0102] So the maximum total lift force on the aileron will be:$\begin{matrix}{\quad {= {{\left( {0.59 \times 1000 \times 1.03 \times 0.0825 \times 0.1389 \times (15.44)^{2}} \right)/\left( {2 \times 9.81} \right)}\quad {kg}}}} \\{= {84.62\quad {{kg}.}}}\end{matrix}$

[0103] If it is assumed that the aileron can be mounted within 5% ofchord length of the centre of lift, then the hydrodynamic out of balancemoment will be: $\quad\begin{matrix}{{= {84.62 \times 138.9 \times 0.05\quad {kg}\quad {mm}}}\quad} \\{{= {0.588\quad {kgm}}}\quad}\end{matrix}$

[0104] The frictional torque required to rotate the aileron about itsshaft approximates to: $\quad\begin{matrix}{\quad {= {{Force} \times {Coefficient}\quad {of}\quad {friction} \times {{Radius}.}}}\quad} \\{= {84.62 \times 0.1 \times 0.012\quad {kgm}}} \\{= {0.102\quad {kgm}}}\end{matrix}$

[0105] If these two torques are now added together, the total torquerequired to drive the aileron is: $\quad\begin{matrix}{= {\left( {0.588 + 0.102} \right) \times 9.81\quad {Nm}}} \\{{= {6.77\quad {Nm}}}}\end{matrix}$

[0106] The motor selected to drive the aileron system has anintermittent torque rating of 14.5 mNm, and a continuous rating of 5.9mNm.

[0107] This is connected to a gear box having a maximum dynamic outputtorque rating of 100 mNm. The reduction ratio will be 5:1, theefficiency of this gear box is not known, but will be assumed to be 80%,so maximum available torque from gear box: $\quad\begin{matrix}{= {14.5 \times 0.8 \times 5\quad {mNm}}} \\{= {58\quad {mNm}}}\end{matrix}$

[0108] Continuous torque rating: $\quad\begin{matrix}{\quad {= {5.9 \times 0.8 \times 5\quad {mNm}}}} \\{= {23.6\quad {mNm}}}\end{matrix}$

[0109] No figures are available for the efficiency of hooks couplings,so for the purposes of this exercise, a very conservative figure of 95%will be assumed for each joint. So torque available to drive the wormwheel; ${Intermittent}\quad \begin{matrix}{{= {58 \times 0.95 \times 0.95\quad {mNm}}}} \\{= {52.3\quad {mNm}}}\end{matrix}$ ${Continuous}\quad \begin{matrix}{{= {23.6 \times 0.95 \times 0.95\quad {mNm}}}} \\{= {21.3\quad {mNm}}}\end{matrix}$

[0110] The worm and wheel gear box has a reduction ratio of 38:1, theefficiency of similar gearboxes is known to be of the order of 60%, inorder to err on the side of safety, it will be assumed that this geararrangement is 50% efficient. So the gear box output will be:${Intermittent}\quad \begin{matrix}{{= {52.3 \times 38 \times 0.5\quad {mNm}}}} \\{= {993.7\quad {mNm}}}\end{matrix}$ ${Continuous}\quad \begin{matrix}{= {21.3 \times 38 \times 0.5\quad {mNm}}} \\{= {404.7\quad {mNm}}}\end{matrix}$

[0111] The wheel of the gear box drives a crank with a 3 mm throw, sothe driving force available at the crank is:${Intermittent}\quad \begin{matrix}{{= {{993.7/3}\quad N}}} \\{= {331.2\quad N}}\end{matrix}$ ${Continuous}\quad \begin{matrix}{{= {{404.7/3}\quad N}}} \\{= {134.9\quad N}}\end{matrix}$

[0112] When the crank is at the position of zero aileron incidence tothe annular aerofoil, it is able to apply a force of 331.2 N to theaileron operating arm, at a radius of 38 mm, so applying a torque to theaileron of: $\quad\begin{matrix}{\quad {= {331.2 \times 0.038\quad {Nm}}}} \\{= {12.58\quad {Nm}}}\end{matrix}$

[0113] As the crank rotates to produce the full incidence of theaileron, the effective throw of the crank reduces as the Cosine of theangle through which it has moved, so that as full incidence isapproached, the mechanical advantage of the crank over the aileronapproaches infinity. As a consequence, a torque of at least 12.58 Nm isalways available to drive the aileron. (Maximum requirement 6.77 Nm).

[0114] The gear manufacturer's literature indicates that the worm andwheel selected are suitable for continuous operation at 760 mNm oftorque. If this limitation is placed on the system, then the maximumtorque which can be applied in the zero incidence position is:$\quad\begin{matrix}{\quad {= {12.58 \times {760/993.7}\quad {Nm}}}} \\{= {9.62\quad {Nm}}}\end{matrix}$

[0115] This is still some 42% more than the maximum calculatedrequirement of 6.77 Nm.

[0116] In order to apply full aileron incidence, the crank will have torotate 95°. In order to rotate the crank 95°, the motor will have torotate: $\begin{matrix}{{= {{95/360} \times 38 \times 5}}\quad} & {revs} \\{= 50.14} & {revs}\end{matrix}$

[0117] At its maximum intermittent load rating, the motor is able torotate at 4000 r.p.m., so the time taken to apply full aileron will be:$\begin{matrix}{{= {{50.14/4000} \times 60}}\quad} & \sec \\\underset{\_}{\begin{matrix}{= 0.76} & {\quad \sec}\end{matrix}} & \quad\end{matrix}$

[0118] Stress Analysis

[0119] The main aileron spar is made from stainless steel tospecification BS S145, this is particularly suitable for thisapplication, as it can be through hardened by a very moderate heattreatment process, which means that the item can be machine finished inthe soft condition, and there will be no distortion on hardening. Thesteel is hardened to about 400-455 Vickers, which is equivalent to 1290N/mm². As it is a hard material it is reasonable to suppose that the 0.2proof stress will be in the region of 80% of this value, or 1032 N/mm².

[0120] If it is assumed that the stress within the material should notreach more than {fraction (2/3)} of the proof stress, then theacceptable stress level for this material will be: $\begin{matrix}{= {1032 \times {2/3}\quad N\text{/}{mm}^{2}}} & \quad \\\underset{\_}{\begin{matrix}{{= 688}\quad} & {\quad {N\text{/}{mm}^{2}}}\end{matrix}} & \quad\end{matrix}$

[0121] The spar is to withstand a central load placed on it of 1 tonne(9810 N). If we consider the spar as two cantilevers protruding from thecentral body, and that the load is applied to the spar at the centre ofthe annular aerofoil thickness, then: $\begin{matrix}{{Maximum}\quad {applied}} & {= {{9810/2} \times \left( {125 - 32} \right)}} & {Nmm} \\{{bending}\quad {moment}} & {= 456165} & {Nmm}\end{matrix}$

[0122] If the spar is considered to be a tube with a 3 mm wallthickness, as it is within part of central body, then: $\begin{matrix}{{{Second}\quad {moment}\quad {of}\quad {area}} = {{pi} \times {\left( {d_{1}^{4} - d_{2}^{4}} \right)/64}}} \\{= {3.142 \times {\left( {24^{4} - 18^{4}} \right)/64}\quad {mm}^{4}}} \\{= {11133.019\quad {mm}^{4}}}\end{matrix}$

[0123] The maximum stress within the material at the point where itenters the central body is given by:$\quad {{Stress} = \frac{\begin{matrix}{{Bending}\quad {moment} \times {distance}\quad {from}} \\{{outermost}\quad {fibre}\quad {to}\quad {neutral}\quad {axis}}\end{matrix}}{{Second}\quad {moment}\quad {of}\quad {area}}}\quad$$\quad \begin{matrix}{= {\left( {456165 \times {24/2}} \right)/11133.019}} & {N\text{/}{mm}^{2}} \\{{= 491.69}\quad} & {\quad {N\text{/}{mm}^{2}}}\end{matrix}$

[0124] This is well within the estimate of the acceptable stress givenabove.

[0125] As can be seen from FIGS. 4 and 5, a marine streamer according tothe invention comprises a streamer 7 of generally conventionalconstruction but characterised by the inclusion of a plurality ofcontrol devices 1 permanently fixed at periodic distances along thelength of the streamer. In FIG. 4, the streamers 7 are wound in a firstconfiguration where lengths of streamer not carrying a control device 1are wound onto a first barrel 11 of a winch 9, while lengths of thestreamer which carry a control device are wound over a second barrel 12of winch 9. The two wound portions of the streamer are separated by aslotted, circular separator 10, the cable of the streamer can be passedthrough the slots of the separator to maintain a compact winding on thewinch.

[0126] In FIG. 5, the entire streamer 7 with control devices 1 is woundon a single barrel of a bobbin 9. This arrangement is likely to be lesscompact in a radial dimension than that of FIG. 4 and more prone totangling, however, it may be advantageous in that it may be quicker towind in and easier to automatically wind in.

[0127] It is to be understood that the foregoing specific descriptionsare purely exemplary and are not intended to be limiting of the truescope of the invention as claimed in the appended claims.

1. A control device for controlling the position of a marine streamercomprising; an annular aerofoil; a mount for mounting the annularaerofoil onto and around the streamer; and control means for controllingthe tilt and/or rotation of the aerofoil whereby to adjust the lateralposition and/or depth of the streamer.
 2. A control device as claimed inclaim 1 wherein the mount comprises a hollow shaft positioned centrallyof the annular aerofoil and is rotatably mountable about a longitudinalaxis of a streamer.
 3. A control device as claimed in claim 1 whereinthe mount is permanently fixed to the streamer and has an outer surfacerotatably mounted about the fixed portion of the mount.
 4. A controldevice as claimed in claim 2 wherein the mount is not concentric withthe annulus of the annular aerofoil.
 5. A control device as claimed inclaim 1 wherein the annular aerofoil is connected to the mount byradially arranged struts positioned between the mount and the annularaerofoil.
 6. A control device as claimed in claim 5 wherein the strutshave a symmetrical aerofoil shaped cross-section.
 7. A control device asclaimed in claim 1 wherein the control means incorporates anelectromechanical motor arranged to adjust the rotational and/or lateralposition of the annular aerofoil.
 8. A control device as claimed inclaim 1 wherein the control means incorporates one or more aileronsrotatably mounted about a diameter or portion of a diameter passingthrough the centre of the mount and connecting the mount to the innersurface of the annular aerofoil.
 9. A control device as claimed in claim8 wherein at least one aileron is rotatably mounted about a pair ofpivot points, which extend radially outwardly from the outer surface ofthe amount and/or radially inwardly from the inner surface of theannular aileron.
 10. A control device as claimed in claim 8 wherein thecontrol means incorporates one or more pairs of ailerons which arerotatably mounted in diametrically opposed positions.
 11. A controldevice as claimed in claim 10 wherein the control means comprises asingle pair of ailerons which are rotatably mounted in diametricallyopposed positions, a first of said ailerons being fixedly connected tothe inner surface of the annular aerofoil and the rotably mounted withrespect to the mount and the second of said ailerons being rotatablymounted with respect to both the mount and the annular aerofoil.
 12. Acontrol device as claimed in claim 8 wherein the control meansincorporates one or more electromechanical motors for controlling therotational position of the ailerons about their axis of rotation.
 13. Acontrol device as claimed in claim 12 wherein there is a plurality ofailerons, and the rotational position of at least one of the aileronsabout its axis rotation is controllable independently of the rotationalposition of the one or more other aileron(s) about their respective axesof rotation.
 14. A control device as claimed in claim 8 wherein theaileron(s) are caused to change their translational position as theannular aileron is rotated about the mount.
 15. (Cancelled) 16.(Cancelled)
 17. (Cancelled)
 18. (Cancelled)
 19. (Cancelled)
 20. Acontrol device as claimed in claim 1 wherein the control meansincorporates one or more ailerons mounted about a diameter or portion ofa diameter passing through the centre of the mount and connecting themount to the inner surface of the annular aerofoil and the leading edgesof the ailerons are directed substantially forwardly of the leading faceof the annular aerofoil.
 21. A marine streamer comprising: an elongate,flexible cable carrying along its length a plurality of hydrophones anda plurality of control devices for controlling the position of themarine streamer in water, the control devices being spaced along thelength of the streamer and permanently mounted thereabout and eachcomprising: an annular aerofoil, a mount for mounting the annularaerofoil onto and around the streamer; and control means for controllingthe tilt and/or rotation of the aerofoil whereby to adjust the lateralposition and/or depth of the streamer.
 22. (Cancelled)
 23. A marinestreamer as claimed in claim 21 wherein the annular aerofoil is mountedabout the streamer with its attachment points as near as possible in theplane of its centre of lift.
 24. A marine streamer as claimed in a claim21 to wherein the control devices are positioned at separations ofbetween both 100 and about 500 metres along the length of the streamer.25. (Cancelled)
 26. A marine streamer as claimed in claim 21 wherein thestreamer is a seismic streamer.
 27. A method for winding a marinestreamer as claimed in claims 21 onto a winch comprising; separating thebarrel of the winch into two barrel portions by means of a circularseparator, the separator having a plurality of slots of dimensionssubstantially similar to the cross section of the narrowest diameter ofthe streamer arranged around its perimeter; winding lengths of thestreamer not carrying a control device about a first portion of thebarrel; and winding portions of the streamer carrying a control deviceabout the second portion of the barrel, whilst passing connectingportions of the streamer through the slots in the separator, thereby toprovide a compact and secure winding of the streamer on the winch. 28.(Cancelled)
 29. (Cancelled)
 30. (Cancelled)